1. Field of the Invention
The present invention relates to coatings for permanent molds used in the casting industry and, more particularly, plasma spray zirconia coatings for permanent molds that may be stabilized with yttria or ceria or an equivalent.
2. Description of the Prior Art
Mold coatings employed in the permanent mold casting industry today generally consist of an insulating coating layer combined with a release coating layer. These coatings are necessary to control the casting solidification rate, casting release, limit metal build-up on the mold surface, and to generally prolong mold life. Unfortunately, the durability of these coatings is quite poor, which necessitates their frequent repair and even total reapplication during a casting campaign. The frequent repair and touch-up of the coatings may affect casting performance and quality through inconsistent heat transfer with variable coating thicknesses and metal build-up on the mold surface. Further, the downtime associated with maintaining the coatings significantly affects productivity. When casting performance is unsatisfactory, the coatings/metal build-up may either be locally removed with an abrasive blast or grinding process or, in extreme cases, the whole cavity must be cleaned and recoated. Finally, since these coatings are normally applied by low technology painting or spraying processes, it is likely that the coatings will vary within a mold or from one application to the next.
In addition, coatings in use today do not perform adequately as they have low wear resistance against aluminum (drag/sliding wear against the casting on ejection), relatively low adhesion to the mold surface, low bond strength between powder particles forming the coatings (low cohesion), and low thermal fatigue resistance. These characteristics result in coatings that fail through a combination of wear mechanisms, principally sliding wear and fatigue.
Known prior art references in this area include U.S. Pat. Nos. 5,464,797 to Yasrebi et al.; 5,394,933 to Takayanagi; 4,877,705 to Polidor; 4,787,439 to Feagin; 4,740,246 to Feagin; 4,196,769 to Feagin; and 4,175,611 to Fletcher. The subject matter of the foregoing references is incorporated herein by reference. The Yasrebi et al. patent discloses yttria-zirconia slurries and mold facecoats for casting reactive metals in ceramic molds. The Takayanagi patent discloses a core for casting titanium and titanium alloys. The Takayanagi patent, in particular, discloses the use of a surface layer made of a material selected from among yttria, zirconia, hafnia, alumina, neodymium oxide, and samarium oxide, which is deposited as a coating layer on the surface of the core. The Polidor patent discloses a method of plasma spray coating ceramic bodies. In particular, the Polidor patent discloses the use of corrosion and erosion resistant materials suitable for such plasma sprayed coatings on alumina-graphite and zirconia-graphite bodies including one or more as mixtures of compounds of the following: zirconia; chromium oxide; alumina; silica; magnesia; zirconium silicate; and titanium oxide. The Feagin patents listed hereinabove are directed to casting of reactive metals in ceramic molds. The Feagin ""769 patent, in particular, discloses a method of casting alloys having directionally solidified grains using a mold with coatings of zirconia and yttria. The Fletcher patent listed hereinabove discloses a graphite die that is plasma flame spray coated with a layer of magnesium zirconate.
In view of the foregoing, there is a need in the permanent mold casting industry to reduce production costs and increase productivity, casting quality, casting consistency, and alloy flexibility in permanent mold castings through the use of a chemically stable and durable coating(s). In addition, there is a need to inhibit or change the wear mechanisms present during casting campaigns utilizing permanent molds. Further, there is a need in the permanent mold casting industry to provide the ability to tailor the thermal properties of the coating(s) applied to the mold surface by changing the material used and the structure of the coating (i.e., number of layers, porosity, thicknesses, etc.).
The above needs are fulfilled with a coating for a mold surface of a casting mold in accordance with the present invention. The coating of the present invention is a chemically stable and durable mold coating having certain desired characteristics including: (1) thermal conductivity similar to known mold coatings; (2) thermal shock resistance; (3) thermal fatigue resistance; (4) wear resistance (i.e., casting drag); (5) molten aluminum dissolution resistance; and (6) reproducibility. The coating according to the present invention is a multi-layered coating for a mold surface of a casting mold, which generally comprises a metallic bond coat for bonding to the mold surface and a topcoat comprising yttria stabilized zirconia applied over the metallic bond coat.
The coating may further comprise an intermediate coat located between the metallic bond coat and the topcoat. The intermediate coat may comprise a graded mixture of yttria stabilized zirconia and NiCr. The topcoat may be applied to the mold surface by thermal plasma spraying. The topcoat is preferably about 0.005 to about 0.03 inch thick. The topcoat preferably has a porosity of about 5 to about 15%. The topcoat may further comprise magnesium zirconate
The coating may further comprise a surface layer having the same composition as the topcoat, preferably with a porosity of less than about 1%. The surface layer may be about 0.001 to about 0.002 inch thick.
The metallic bond coat is preferably about 0.002 to about 0.005 inch thick. The metallic bond coat may comprise NiCr or NiCrAlY.
In another embodiment, the multi-layered coating for the mold surface may comprise a metallic bond coat for bonding to the mold surface, a topcoat, and an intermediate coat located between the metallic bond coat and the topcoat. The topcoat preferably comprises yttria stabilized zirconia applied over the metallic bond coat. The topcoat preferably has a porosity of about 5 to about 15%. A surface layer is applied over the topcoat. The surface layer preferably comprises the same composition as the topcoat and has a porosity of less than about 1%. The intermediate coat preferably comprises a graded mixture of the materials comprising the metallic bond coat and the topcoat.
The metallic bond coat preferably comprises NiCr or NiCrAlY. The topcoat is preferably applied to the mold surface by thermal plasma spraying. The topcoat may further comprise magnesium zirconate.
The present invention is further directed to a method of coating a mold surface of a casting mold, generally comprising the steps of: providing a casting mold having a mold surface, with the mold surface shaped to form a cast product; applying a metallic bond coat to the mold surface; and applying a topcoat over the metallic bond coat.
The method step of applying a topcoat may comprise thermal plasma spraying yttria stabilized zirconia over the metallic bond coat. The method may further comprise a step of plasma spraying an intermediate coat on top of the metallic bond coat prior to the step of plasma spraying the topcoat. The intermediate coat may comprise a graded mixture of the materials comprising the metallic bond coat and the topcoat. The metallic bond coat may comprise NiCr or NiCrAlY.
The topcoat preferably has a porosity of about 5 to about 15%. The method may further comprise a step of applying a surface layer over the topcoat. The surface layer preferably has the same composition as the topcoat and a porosity of less than about 1%.
Further details and advantages of the present invention will become apparent from the following detailed description when read in conjunction with the drawings, wherein like parts are designated with like reference numerals.